The heart of a computer is a magnetic hard disk drive (HDD) which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The volume of information processing in the information age is increasing rapidly. In particular, HDDs have been desired to store more information in its limited area and volume. A technical approach to this desire is to increase the capacity by increasing the recording density of the HDD. To achieve higher recording density, further miniaturization of recording bits is effective, which in turn typically requires the design of smaller and smaller components. One approach to achieve this reduction in component size is to use a magnetoresistive (MR) read head having a spin valve element.
A MR read head comprising a spin valve element is generally used for a read head employed in a HDD. A hard bias read head or reader in which a hard magnetic material is provided in the track width direction of the spin valve element, and a side shield read head (or a side shielded reader, as described in U.S. Patent Application Publication No. 2012/0087045) in which a soft magnetic material is provided instead of a hard magnetic material are known in the art.
In a side shield read head, the sensitivity at the edges in the read sensitivity distribution is reduced as a result of the soft magnetic material being provided in the track width direction of the spin valve element. This reduction in sensitivity at the edges of the sensitivity distribution is due to the spin valve element attracting the magnetic field generated in the central portion of the recording track, while the soft magnetic material magnetic shield absorbs the magnetic field generated at the periphery of the track width. It is possible to reduce read-out noise at the track ends and interference from adjacent tracks as a result of the reduction in sensitivity at the edges, and therefore it is possible to increase the track density. Side shield read heads have therefore have become more popular in recent years.
Reducing the geometric dimensions of the actual spin valve element is also effective for improving the track density. Reducing the dimensions of the element in the track width direction facilitates reading-out of magnetic information in recording tracks having a narrow recording width. Currently, the dimensions of a spin valve element are below about 50 nm.
However, new problems have arisen as spin valve elements have become smaller. This includes variations in the characteristics of individual read heads. There are fluctuations (production variations) in the film-forming conditions of the film-forming process for spin valve elements, etc. These fluctuations affect the shape, size, and film characteristics of the elements, and are manifested as variations in the read characteristics of the spin valve element. One effect of the fluctuations becomes relatively larger as the size of the elements decreases.
Head signal-to-noise ratio (SNR) is one read characteristic. Head SNR is defined as the ratio of the magnitude of the output of read signals in a low-density recording pattern to the head noise caused by the read head. In general, the greater the head SNR, the more accurately recorded information is able to be read out from the medium.
FIG. 5 shows the relationship of head SNR and read utilization for a plurality of read heads produced under the same conditions. Here, read utilization is defined as the ratio of the amount of variation in resistance when a medium field is applied to individual spin valve elements to the maximum amount of variation in resistance. The magnitude of the read output is proportional to the magnitude of the read utilization, and therefore there is a very close relationship between head SNR and read utilization. In FIG. 5, the read utilization should be around 35% in order to achieve the best head SNR. However, the utilization of individual heads is scattered in a range between less than 25% and 45%, so not all of the heads are optimized. Also, the head SNR decreases outside the optimum range and therefore, any read operations to read recorded signals may be inadequate. The proportion of products which operate in accordance with the specification is referred to as the yield. Variations in read utilization may cause a reduction in yield so steps should be taken against this.